As I told someone else, girls are very visual and action oriented. You can say it, and there can be all the trust in the world, but if you're going around hitting on people or making propositions or whatnot, a girl is going to feel insecure. Of course, it's not warranted, and I can understand the frustration.
Try sitting down and talking to her. You shouldn't be expected to completely change who you are, but if she is feeling like she's not good enough, a compromise has to be made between both of you. A little reassurance, plus maybe a few minor behavioral changes, should help. And of course, communication!